Many people use wireless communication systems. In wireless communication systems the information is typically transmitted by electromagnetic waves traveling through the air or other medium. One form of wireless communication is spread-spectrum communication. Spread-spectrum communication systems use a synchronized version of a spreading code to demodulate the received signal. Spreading codes will be discussed below. Typically, the synchronizing process between the locally generated spreading signal and the received spread spectrum signal happens in two parts. The first part is acquisition. During acquisition both signals are coarsely aligned with each other. In other words, the spreading code is rough alignment in time. The spreading code is roughly synchronized. After this coarse alignment the second part, tracking makes fine alignments to maintain the best waveform possible.
The acquisition phase involves searching through time and frequency uncertainty to synchronize a received spread-spectrum signal with a spreading code that has been generated locally. Many uncertainties exist during acquisition. Uncertainty in the distance between the transmitter and receiver of a spread-spectrum communications system leads to uncertainty in the amount of delay between the transmitter and receiver. Delay as a signal travels, for example from a transmitter to a receiver, is known as propagation delay. This can happen between any transmitter and receiver. Several examples are given using a base station and a mobile station.
An example of a spreading code and uncertainty in the distance between the transmitter and receiver will now be discussed. A spreading code is a pseudo-random sequence. The spreading code would typically be used to modulate a spread-spectrum signal at a base station for transmission to a mobile station. Additionally, the spreading code would typically be used to demodulate signals received from the mobile station. A locally generated spreading code 456 will typically be used to demodulate the received signal from the base station and to modulate signals transmitted to the base station.
A locally generated spreading code and the spreading code at the base station are not synchronized initially. One thing that increases the difficulty of synchronizing these two signals, as discussed above, is delay as the signal travels from the base station to the mobile station. Spreading codes will be discussed further with respect to FIG. 10 below.
Additionally, uncertainty regarding a receiver's velocity relative to a transmitter leads to uncertainty regarding Doppler frequency shift of the transmitted signal. A Doppler shift is the change in apparent frequency of a source of electromagnetic radiation or sound when there is a relative motion between the source, in this case the transmitter, and the observer, in this case a mobile handset that is attempting to synchronize with a spread-spectrum communications network.